An electronic throttle control system (ETC) for controlling the amount of air to an engine is well known. The throttle valve plate of the throttle body pivots in response to signals received from the engine control unit (ECU) and thereby acts to regulate the amount of air delivered to the intake manifold. Throttle position sensors maintain the correct throttle position over the range of expected engine torque loads.
Prior art throttle plates are typically designed to default to a slightly open position (e.g., by about 10-20 degrees depending on the particular application) when in the engine-off condition or upon receiving an ETC fault signal. This is because there exists the possibility of the throttle plate becoming stuck in the fully closed position due to winter-time ice or carbon build-up in this area. Also, should the ETC malfunction, some air must be able to reach the intake manifold to allow the car a “limp home” condition for needed repairs. Thus, the prior art typically provides an ETC throttle system with a throttle plate that defaults to an open position in engine-off and ETC failure conditions. However, one serious drawback to a throttle open default position is that hydrocarbons remaining in the intake manifold are allowed to evaporate and escape into the atmosphere. Present day emissions regulations require throttle designs which substantially restrict the escape of hydrocarbons which would otherwise occur through an open throttle plate during engine-off or ETC failure conditions. Therefore, there exists a need for an ETC throttle body that gives the engine a limp home condition yet restricts the escape of evaporated hydrocarbons during engine-off and ETC failure conditions.